1. Field of the Invention
The present invention relates to radar timing circuits, and more particularly to precision swept delay circuits for equivalent time ranging systems. It can be used to generate a swept-delay clock for sampling-type radar, TDR and laser systems.
2. Description of Related Art
High-resolution pulse-echo systems such as wideband pulsed radar, pulsed laser rangefinders, and time domain reflectometers generally sweep a timing circuit across a range of delays. The timing circuit controls a receiver sampling gate such that when an echo signal coincides with the temporal location of the sampling gate, a sampled echo signal is obtained. The echo range is then determined from the timing circuit, so highly accurate timing is needed to obtain accurate range measurements.
One approach to generate swept timing employs open or closed-loop analog techniques: (1) open-loop circuits generally use an analog voltage ramp to drive a comparator, with the comparator reference voltage controlling the delay, and (2) closed-loop timing circuits generally employ a phase-locked loop (PLL), wherein the phase difference between a transmit and a receive clock is measured and controlled with a phase comparator and control loop. Both architectures have their limitations--open-loop circuits are subject to component and temperature variations, and are not very accurate due to the difficulty in generating a precision voltage ramp with sub-nanosecond accuracy; and closed-loop circuits rely on analog component ratios to set the accuracy. Examples of closed-loop architectures are disclosed in U.S. Pat. No. 5,563,605, a "Precision Digital Pulse Phase Generator" by McEwan, and in copending application, "Phase-Comparator-Less Delay Locked Loop", Ser. No. 09/084,541, by McEwan. The present invention significantly improves upon the accuracy of analog swept delay circuits by eliminating the accuracy-limiting analog components altogether.
Another approach to generate swept timing employs two oscillators with frequencies F.sub.T and F.sub.R that are offset by a small amount F.sub.T -F.sub.R =.DELTA.. In a radar application, a transmit clock at frequency F.sub.T triggers transmit RF pulses, and a receiver clock at frequency F.sub.R gates the echo RF pulses. If the receive clock is lower in frequency than the transmit clock by a small amount .DELTA., it will smoothly and linearly slip in phase relative to the transmit clock such that one full cycle is slipped every 1/.DELTA. seconds. Typical figures are: transmit clock F.sub.T =1 MHz, receive clock F.sub.R =0.9999 MHz, .DELTA.=100 Hz, and slip period=1/.DELTA.=10 milliseconds.
The receive gate samples the radar echoes and produces an output voltage that varies with the phase of the receive clock relative to the transmit clock (and the radar echoes). This variation occurs on a 10 ms scale, and represents events occurring on a 1 .mu.s scale. The corresponding time expansion factor is 10 ms/1 .mu.s=10000. Thanks to this expansion effect, events on a 10-picosecond scale are converted to an easily measurable 100-nanosecond scale. In contrast, a real time counter would need a 100 GHz clock to measure with 10 ps resolution, well beyond present technology.
This two-oscillator technique was used in the 1960's in precision time-interval counters with sub-nanosecond resolution, and appeared in a short-range radar in U.S. Pat. No. 4,132,991, "Method and Apparatus Utilizing Time-Expanded Pulse Sequences for Distance Measurement in a Radar," by Wocher et al.
The accuracy of the two-oscillator technique is limited by the accuracy of the clocks, which can be extremely accurate, and by the smoothness and linearity of the phase slip between them. No means or data appears in the prior art to support the accuracy of the phase slip--it is not easy to measure, and it is also easy to assume it is somehow perfect. Unfortunately, there are many influences that can affect the smoothness of the phase slip that are addressed by the present invention. These include digital cross-talk that can produce 100 ps error or more, and offset frequency control circuit aberrations than can introduce even more substantial phase slip nonlinearities.
A significant drawback with the two-oscillator technique is that the phase slips over a full 360 degrees of the transmit clock. Ideally, a radar system will gate over perhaps the first 36 degrees after a transmit pulse. The remaining 324 degrees is dead time to allow for distant echoes to diminish before the next cycle. If this dead time is too short, range ambiguities will result. Thus, the receiver in the two-oscillator systems spends 90% of its time phase slipping over out-of-range, potentially ambiguous echoes. In effect, 90% of the transmitted pulses are wasted. What is needed is a two-oscillator timing system that phase slips over only the first .about.36-degrees, coupled with a system that resolves crosstalk and control errors. In addition, a low cost implementation is essential to the commercial success of the innumerable non-contact ranging applications based on the present invention.